For example, in accordance with increase of the demand for large-capacity data transmission, the research and the development of a digital coherent optical transmission system that enables data transmission more than 100 (Gbps) by a single wavelength light are performed. In the digital coherent optical transmission system, not only an intensity of light but also a phase of the light is used for signal modulation, unlike an intensity modulation system. For example, DP (Dual-Polarization)-QPSK (Quaternary Phase Shift Keying) is mentioned as such a modulation system.
In the Wavelength Division Multiplex (WDM), there is used an optical signal formed to a spectral waveform (hereinafter referred to as “Nyquist waveform) satisfying a Nyquist first reference which is a condition for suppressing inter-code interference of a signal in order to effectively utilize a frequency band (see e.g. Japanese Laid-open Patent Publication No. 2014-72824). When the optical signal having the Nyquist waveform is transmitted by the digital coherent optical transmission system, a transmitting device of a reception side photoelectrically converts a received optical signal into a digital signal, and then performs spectral shaping on the digital signal with the use of a digital filter, such as a FIR (Finite Impulse Response) filter, in order to improve reception characteristics.